Many telephone systems, such as the AT&T MERLIN.RTM. and the AT&T System 75.RTM. telephone systems, use a limited number of outside telephone lines to service a large number of client telephones. For example, a telephone system switch may connect with four outside telephone lines, but have a capacity to handle ten or more internal client telephones. Client telephones can be fully digital, hybrid or analog. In a digital client telephone, all data and voice information is sent and received in binary data packets along a single pair of digital cables. A hybrid client telephone sends and receives voice information across an analog pair of cables and data across a digital pair of cables. An analog client telephone sends and receives data and voice information across a single analog pair of cables.
Recently, adjunct processors have been developed which connect to telephone systems and increase the system's capabilities. Examples of adjunct processors are voice mail, automated attendant and interactive voice response. Adding an adjunct processor to an existing telephone system allows upgrading and modernization without replacing the entire telephone system, usually a more expensive option.
The functions of many adjunct processors are controlled by receipt of certain signals. In many instances, this control signal is a dual-tone multi-frequency ("DTMF") signal. A DTMF signal can be represented in analog wave form or as a digital data packet. In a telephone system, a client telephone signals the telephone system to generate a DTMF signal when the client telephone's key pad (0-9 plus the # and * keys) is pressed. Many adjunct processors use a DTMF signal as a control means because a DTMF signal's primary function is to "dial" by indicating a desired number. Therefore, most telephones have DTMF signal generation capabilities.
In many telephone systems, the client phones are incapable of independently generating a DTMF signal; the switch possesses a central DTMF signal generator accessed by each client telephone when an outside line is requested. This arrangement reduces the cost of the telephone system by centralizing DTMF signal generation, while still allowing outside dialing and manipulation of adjunct processors connected to other telephone systems. However, this arrangement also forces connection of all desired adjunct processors controlled by a DTMF signal to the telephone system at an outside line port. In effect, the switch recognizes the request from the client telephone for access to an adjunct processor as a request for access to another telephone system. Thus, the request from the client telephone activates the switch's DTMF signal generator, as if the adjunct processor is a device external to the telephone system. This solution has been favored because altering the internal logic of a telephone system switch is expensive, and in many instances, voids warranties on the telephone system itself.
Although connecting adjunct processors through an outside line has been successful, it does so by permanently tying up an outside line, thus, reducing the system's ability to service its client telephones.
A more practical solution is to attach the adjunct processor to a client port of the telephone system switch. However, many telephone systems only allow access to its DTMF generator when an outside line is requested by the client telephone and not when access to another client port is requested.